Over the past decade, chlorinated poly(vinyl chloride) resin (hereafter "CPVC" for brevity), has moved into the vanguard of vinyl chloride resins because of its unique suitability for rigid and semi-rigid compositions used in extruded pipe, cable jacketing and structural components for buildings. The chlorination of various types of poly(vinyl chloride) resins (hereafter "PVC" for brevity), by different methods is disclosed in the textbooks "Polyvinylchloride and Vinylchloride-Mischpolymerizate," pp. 120-125, Springer, Berlin (1951), by C. A. Schildknecht (1952); and also in U.S. Pat. Nos. 2,426,808, 2,590,651 and 2,996,489 (hereafter "the '489 process," for brevity), inter alia.
The process of the present invention permits the chlorination of PVC in water without the use of swelling agents. The use of swelling agents is taught in the '489 process the disclosure of which is incorported by reference thereto as if fully set forth herein. The '489 process produces excellent quality CPVC except that the CPVC must be freed from residual swelling agents, as described for example, in U.S. Pat. No. 4,147,859. Also, the CPVC is produced more slowly, and is therefore less economical. As is wellknown in the art, due to the relatively high cost of PVC and the expense of chlorinating the PVC, it is essential that the cost of converting it to CPVC be minimized if the CPVC product is to be an affordable commodity. Hence the intense concentration in the art to develop an improved process.
The art has long recognized the problem of relatively slow chlorination of vinyl chloride resins and much effort has been expended to overcome this problem without sacrificing the quality of the CPVC. For example, U.S. Pat. No. 3,100,762 to Shockney (hereafter "the '762 process" for brevity) describes obtaining faster chlorination than in the '489 process by conducting the chlorination at elevated temperature and pressure in the presence of a swelling agent, but in the absence of photo-illumination. It is taught therein that no catalyst, and particularly no photo-illumination is required under the improved conditions of temperature in the range from about 60.degree. C. to about 100.degree. C., and a reactor pressure in the range from about 20 to about 80 psig, if oxygen is substantially excluded from the reactor, but that inferior chlorinated products are obtained under the foregoing reaction conditions when the chloromethane swelling agent is omitted from the reaction mixture.
The '762 process failed to discover the critical importance of photo-illumination, though it may be stated that the criticality of photo-illumination was overlooked because of the presence of swelling agent, which was deemed essential. Yet, it must also be recognized that the '489 process taught that with photo-illumination in the presence of a swelling agent, temperatures not higher than 65.degree. C. were to be used. The drawbacks of the '762 process were that (a) it required a swelling agent, (b) it maintained constant temperature, and most important, (c) the CPVC product had a heat distortion temperature ("HDT" for brevity), of less than 115.degree. C. It has now been found that with a particular set of process conditions, specified in the instant invention, the swelling agent may be omitted, and at the same time, the quality of the product and the economic attractiveness of the process are improved.
A succession of concerted efforts have been directed to the development of a process for the water chlorination of PVC without the use of swelling agents. For example, U.S. Pat. No. 3,506,637 teaches the use of especially prepared PVC which is chlorinated in the presence of a controlled supply of oxygen in the absence of swelling gents. U.S. Pat. No. 3,534,013 to Wakbayashi et al. also teaches the chlorination of a specially prepared PVC which is prepared by suspension of emulsion polymerized vinyl chloride monomer in the presence of a chlorinated lower alkane.
The relative difficulty with which an aqueous suspension of PVC may be thermally chlorinated is well known, and the product obtained is generally poor in quality and commercially unacceptable for use, esepcially in extruded articles. As disclosed in U.S. Pat. No. 3,632,848 to Young et al., the rate of thermal chlorination of an aqueous suspension of PVC is improved when chlorination is initiated at or above 100.degree. C. and below 140.degree. C. while the suspension of PVC is purged with nitrogen to remove oxygen. This teaching that the critical temperature range for chlorination in the Young et al. process is necessarily above the glass transition temperature ("T.sub.g ") of the PVC resin has now been found to be injurious to the quality of the CPVC as produced in our process where the temperature at which an aqueous suspension of PVC is chlorinated is no higher than the T.sub.g f the PVC resin, and always remains below the effective T.sub.g of the resin mass during the chlorination. Though the temperature at which chlorination was initiated, in the Young et al. process then necessary under their process conditions, it was so high as to preclude the use of photo-illumination and require the use of a particular metallic ion catalyst, and these conditions adversely affected both the rate and the quality of the CPVC produced, as compared to the rate and quality of CPVC produced by the instant invention.
Another process disclosed in British Pat. No. 1,186,847 to Solvay & Cie teaches chlorination of PVC made by an acetyl peroxide catalyzed reaction. An aqueous suspension of this PVC is stirred continuously and chlorine is blown in under an effective pressure of 2-10 kg/cm.sup.2, preferably 4-5 kg/cm.sup.2, while continuously raising the temperature of the reaction medium from 35.degree. to 80.degree. C. However, as will be illustrated hereafter, the amount of the chlorine in the CPVC fails to reach 67% even after 10 hours, and the process is too slow to be commercially advantageous.
Still another process, disclosed in U.S. Pat. No. 4,049,517 to Adachi (hereafter "the '517 process" for brevity) teaches varying the amount of ultraviolet radiation (referred to as "light ramping") during the chlorination reaction to control the reaction rate within predetermined limits, thus avoiding the use of a swelling agent for the PVC, but failed to discover the importance of controlling reaction temperature by autogenously increasing it (referred to as "autogenous temperature ramping" because of the self-induced increase of temperature due to the exothermic reaction) if the pressure is maintained at a level in the range from about 10 psig to about 100 psig, or even higher, and, the level of radiation is maintained at a predetermined level.
The importance of autogenous temperature ramping, was overlooked in the '517 process because of the expected relationship of pressure and temperature on gases as evident by their teaching that, even if the amount of the dissolved Cl.sub.2 is increased to several times as much as that under atmospheric pressure, the reaction rate under pressure is almost the same as that under atmospheric pressure, when the same quantity of ultraviolet light irradiates a unit amount of PVC. This finding led to the conclusion that Cl.sub.2 was not insufficient in the reaction under atmospheric pressure. This conclusion has been found to be contradicted by the process of the instant invention because it is now evident that elevated pressure must be used.
Though each of the aforementioned references recognized the practical onus of a determinedly slow chlorination reaction for PVC, and some indicate the desirability of leaving out the swelling agent, none indicates that the swelling agent could be left out if a particular combination of elevated pressure, ramped temperature commencing at or below the T.sub.g of the PVC resin, absence of oxygen, and a substantially constant amount and intensity of ultraviolet-illumination was employed. No reference teaches that such a combination of process conditions would increase the concentration of Cl.sub.2 in the aqueous phase so significantly that the known rate-limiting effect of low Cl.sub.2 concentration would be counteracted. Nor was it to be expected that the rate of conversion of PVC to CPVC in the aqueous phase, would be such that, upon the autogenous ramping of temperature in the reactor, the pressure does not get ramped significantly. Further, though it was known that higher porosity of PVC increases the rate of chlorination it was also known that higher pressures produced lower quality, if not unstable, CPVC product. Still further, no reference suggests that a water chlorination of PVC without the use of swelling agents, may provide the rates specified in this invention, without sacrificing the quality, and particularly the heat stability of the CPVC produced.